Engine-out exhaust frequently includes byproducts of combustion that may be detrimental to the environment, and as such are subject to emission regulation. To reduce these so-called regulated emissions, internal combustion engines are frequently equipped with various exhaust-gas aftertreatment systems. For the oxidation of unburned hydrocarbons (HC) and carbon monoxide (CO), an oxidation catalytic converter is often provided in the exhaust-gas discharge system, in particular in the case of internal combustion engines which are operated with an excess of air, for example spark-ignition engines which operate in a lean-burn mode or direct-injection diesel engines.
In the case of spark-ignition engines, use may also be made of three-way catalytic converters, which however provide optimal conversion at stoichiometric operation (λ≈1) within narrow limits. Here, the nitrogen oxides NOx are reduced by the non-oxidized exhaust-gas components which are present, specifically the carbon monoxides and the unburned hydrocarbons, wherein said emissions are oxidized at the same time.
With an excess of air (λ>>1), the nitrogen oxides in the exhaust gas cannot be reduced out of principle, that is to say on account of the lack of reducing agent. To reduce the nitrogen oxides, use is therefore made of selective catalytic converters—so-called SCR catalytic converters—in which reducing agent is purposely introduced into the exhaust gas in order to selectively reduce the nitrogen oxides. As reducing agent, in addition to ammonia and urea, use may also be made of unburned hydrocarbons. The latter is also referred to as HC enrichment, with the unburned hydrocarbons being introduced directly into the exhaust-gas discharge system or else by engine-internal measures, for example by a post-injection of additional fuel.
The nitrogen oxide emissions may also be reduced by a so-called nitrogen oxide storage catalytic converter (LNT—Lean NOx Trap). Here, the nitrogen oxides are initially—during a lean-burn mode of the internal combustion engine—absorbed, that is to say collected and stored, in the catalytic converter before being reduced during a regeneration phase, for example, by substoichiometric operation (for example λ<0.95) of the internal combustion engine with a lack of oxygen. During the regeneration phase, the nitrogen oxides NO are released and converted substantially into nitrogen dioxide (N2), carbon dioxide (CO2) and water (H2O).
To minimize the emissions of soot particles, use is made of so-called regenerative particle filters which filter the soot particles out of the exhaust gas and store them, with said soot particles being burned off intermittently during the course of the regeneration of the filter.
One difficulty with the use of an LNT results from the sulfur contained in the exhaust gas, which is likewise absorbed in the LNT. The high temperatures used for a desulfurization can damage the storage catalytic converter, contribute to thermal aging of the catalytic converter and adversely affect the desired conversion of the nitrogen oxides. The storage capacity of the LNT, that is to say the capability thereof to store nitrogen oxides, decreases with advancing operating duration.
The high exhaust-gas temperatures lead to thermal aging and, with advancing operating duration, to a restriction of functionality, that is to say a decrease in conversion rates, not only in the case of a storage catalytic converter but also in the case of an oxidation catalytic converter. In particular, the high temperatures used for the oxidation of methane have proven to be critical.
On account of the fact that the efficiency of an exhaust-gas aftertreatment system decreases with advancing operating duration or an exhaust-gas aftertreatment system can basically also be irreversibly damaged, an exhaust-gas aftertreatment system or the functionality of such a system may be monitored in order to prevent undesirably high emissions as a result of restricted functionality or lack of conversion. Even though the present regulations do not imperatively require on-board diagnosis (OBD), future concepts may necessitate this. For example, the EURO VI regulation prescribes the monitoring of nitrogen oxide untreated emissions.
One engine-internal measure for the reduction of nitrogen oxide emissions includes exhaust-gas recirculation, that is to say the recirculation of exhaust gases from the exhaust-gas discharge system into the intake system via a recirculation line.
Exhaust-gas recirculation is a concept for reducing nitrogen oxide emissions, wherein the nitrogen oxide emissions can be reduced considerably with increasing exhaust-gas recirculation rate. Here, the exhaust-gas recirculation rate xEGR is determined as follows:xEGR=mEGR/(mEGR+mFresh air)where mEGR denotes the mass of recirculated exhaust gas and mFresh air denotes the supplied fresh air, that is to say combustion air, which has if appropriate been delivered and compressed by means of a compressor.
Exhaust-gas recirculation is also suitable for reducing the emissions of unburned hydrocarbons in the part-load range. To obtain a considerable reduction in nitrogen oxide emissions, high exhaust-gas recirculation rates may be used, such as of the order of magnitude of xEGR≈60% to 70%.
In the case of internal combustion engines which are supercharged by exhaust-gas turbocharging and which are equipped with an exhaust-gas recirculation system, this results in a conflict because the recirculated exhaust gas is generally extracted from the exhaust-gas discharge system upstream of the turbine of the at least one exhaust-gas turbocharger and is no longer available for driving the at least one turbine.
In an exhaust-gas turbocharger, a compressor and a turbine are arranged on the same shaft, with the hot exhaust-gas flow being supplied to the turbine and expanding in said turbine with a release of energy, as a result of which the shaft is set in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor delivers and compresses the charge air supplied to it, as a result of which supercharging of the internal combustion engine is obtained.
In the event of an increase in the exhaust-gas recirculation rate, the exhaust-gas flow conducted through the turbine decreases. The reduced exhaust-gas flow through the turbine leads to a lower turbine pressure ratio, with which the charge pressure ratio also falls, which is equivalent to a smaller compressor mass flow.
The described effects, that is to say both the increase in the exhaust-gas recirculation and also the simultaneous decrease in the charge pressure caused by this, lead to a richer cylinder fresh charge, that is to say to less fresh air or oxygen in the combustion chamber, which has a significant influence on the formation on the emissions and the emission concentrations in the exhaust-gas discharge system.
The inventors have recognized the issues with the above approaches and offer a method herein to at least partly address them. A method for monitoring a regulated emission concentration Ci in the exhaust gas of an internal combustion engine is provided. The method comprises directing the exhaust gas through an exhaust-gas turbocharger, directing at least a portion of the exhaust gas through an exhaust-gas recirculation system, measuring an air ratio λmeas in the exhaust gas with a lambda probe, measuring a rotational speed nT of the exhaust-gas turbocharger with a sensor, and determining the emission concentration Ci based on the air ratio λmeas and the rotational speed nT.
In this way, an emission concentration C, in the exhaust gas of the internal combustion engine can be more accurately determined by taking into account the reduction in mass flow of the exhaust brought about by the EGR system. By doing so, a more inexpensive sensor can be utilized with a more robust determination of the emission concentration of the exhaust.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.